Solid-acid fuel cells (SCFCs) utilize an anhydrous, nonpolymeric proton conducting electrolyte that can operate at slightly elevated temperatures. By supporting thin CsH2PO4 electrolyte membranes (25-36 mu m), on porous stainless steel gas-diffusion electrodes, SAFCs with peak power densities as high as 415 mW/cm(2) were obtained. Cells were operated at similar to 240 degrees C with humidified H-2 supplied to the anode and humidified O-2 supplied to the cathode. Despite the thinness of the membranes, the open-circuit voltages were high, 0.91-1.01 V. These results transform SAFCs from laboratory curiosities into highly competitive energy conversion devices. (C) 2005 The Electrochemical Society.